I have a platinum-iridium out of my old Lincoln LS kicking around on my desk that was changed at ~125K miles. It gaps at .050, and the spec gap is .044. I'd have put it back in had it not been for the fact that I had at least one bad coil, wasn't sure which it was so on advice went ahead and did all 8(it wasn't much more work than changing the front most ones on each side, which would have been the best guess of the bad ones), and changing the plugs added about 5 minutes onto each coil swap. I did notice a difference in power, idle smoothness, and overall driveability when I put it back together, but I attribute that more to the intermittently failing coil being replaced than the plugs themselves.
I've spent some time looking at this particular plug under a microscope, and it's also interesting to me to see how it has worn. The ground strap is badly eroded and pitted across a lot of the surface, but there's a thin strip-presumably a platinum wire-that looks virtually brand new. Similarly, the electrode tip is a bit less pointy/more rounded off than a new plug, which is where I suspect most of the gap loss is from, but it overall looks pretty good.
I may pre-emptively do the MKZ when I hit 100K, but I'll weigh my options on it and see how difficult the job is(I know a lot of transverse Vs can be difficult on the back side) and may defer doing it until I see a good reason to. If I have to do it, I'll do iridiums.
The old British stuff get good old fashioned plain copper NGK BP6ESs. I've never seen any benefit from Platinum, while a mis-adjusted carb can foul them up easily, and they're easy to change. I know people who swear by Champions, which were the OEM supplier, but I've been less than happy with the quality on recent Champions. I have probably a dozen sets of the NGKs put back as I hear intermittent rumors about them being discontinued, they can be hard to find in stock, and they can make a good ~$8 filler at parts houses that I know carry them if I need to push my bill up a few bucks for a special or whatever. As a side note, if you look up year/make/model on my stuff, most of the parts house computers will return the resistor version of the plug(BPR6ES) which I don't like since I always run resistor wires. I've found myself with a blank stare at stores before when I ask for the plug by P/N instead of supplying year/make/model.